decoder_api: automatically send stream tag

If an input stream provides tags (e.g. from an icecast server), send
them in the decoder_data() and decoder_tag() methods.  Removed the
according code from the mp3 and oggvorbis plugins - decoders shouldn't
have to care about stream tags.

This patch also adds the missing decoder_tag() invocation to the mp3
plugin.

src/decoder_thread.c

@@ -72,6 +72,7 @@ static void decodeStart(void)
 	}
 
 	decoder.seeking = false;
+	decoder.stream_tag_sent = false;
 
 	dc.state = DECODE_STATE_START;
 	dc.command = DECODE_COMMAND_NONE;